Composite filter

ABSTRACT

A composite filter downsized without degrading its characteristics is disclosed. The filter includes a surface acoustic wave filter and a LC filter coupled to the surface acoustic wave filter. The LC filter is formed by combining a π-shaped LC filter formed of two capacitors and an inductor coupled together in a π-shape with a capacitor coupled in parallel to the inductor. The two capacitors of the π-shaped LC filter are placed on a piezoelectric substrate with their comb-shaped electrodes opposed to each other. This opposing direction differs from an opposing direction of comb-shaped electrodes of a surface acoustic wave resonator.

TECHNICAL FIELD

The present invention relates to a composite filter to be used incommunication devices such as mobile phones.

BACKGROUND ART

A conventional composite filter used in portable wireless communicationdevices such as mobile phones is described with reference to FIG. 9,which shows its perspective exploded view. In FIG. 9, the conventionalcomposite filter is formed of a combination of elastic surface-wavefilter and a low-pass filter.

The low-pass filter is formed by combining an inductor with a capacitor,and is embedded in laminated substrate 2 made of low temperatureco-fired ceramics and formed by layering a plurality of dielectriclayers 1. The inductor is formed of a plurality of dielectric layers 1with arc-shaped conductors 3 formed thereon and coupled to each other.The capacitor is formed of planar capacitor electrodes 4 sandwichingdielectric layer 1.

The elastic surface-wave filter is formed by combining elasticsurface-wave resonators 5 in a ladder shape as shown in FIG. 10, andplacing resonators 5 on laminated substrate 2.

The low-pass filter is coupled to a transmission output side of theforegoing filter in order to attenuate the doubled or tripled harmonicsin the pass band of the filter. The composite filter discussed above,however; is formed by embedding the low-pass filter in laminatedsubstrate 2, so that the inductor and the capacitor are obliged tooccupy a large area, and the capacitor, in particular, takes a greaterportion of the occupied area because its capacitance is proportionate toits area. As a result, the filter cannot be further downsized.

The related art to the present invention are listed here as patentdocuments 1 and 2.

Patent Document 1: Unexamined Japanese Patent Publication No.2004-254257

Patent Document 2: Unexamined Japanese Patent Publication No.2005-184773

DISCLOSURE OF INVENTION

The present invention addresses the foregoing problem and aims toprovide a composite filter downsized from the conventional one withoutdegrading the characteristics. To achieve this objective, the presentinvention employs the following structure: at least one of capacitors isplaced such that its comb-shaped electrodes are oppositely placed toeach other on a piezoelectric substrate, and the opposing direction ofthe comb-shaped electrodes differs from that of comb-shaped electrodesof a surface acoustic wave resonator.

The foregoing structure allows decreasing an occupied area by thecapacitors because at least one of the capacitors is formed such thatits comb-shaped electrodes oppose to each other on the piezoelectricsubstrate. On top of that, unlike the comb-shaped electrodes of thesurface acoustic wave resonator, the structure allows the comb-shapedelectrodes of the capacitors not to excite because its opposingdirection differs from that of the comb-shaped electrodes of the surfaceacoustic wave resonator, so that no loss is produced. As a result, adownsized composite filter can be obtained without degrading the

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a composite filter in accordance with anexemplary embodiment of the present invention.

FIG. 2 shows an exploded perspective view of the composite filter inaccordance with an embodiment of the present invention.

FIG. 3 shows an equivalent circuit diagram of the composite filter inaccordance with an embodiment of the present invention.

FIG. 4 shows frequency characteristics of the composite filter inaccordance with an embodiment of the present invention.

FIG. 5 shows a top view of a composite filter in accordance with anotherexemplary embodiment of the present invention.

FIG. 6 shows an equivalent circuit diagram of a surface acoustic waveresonator.

FIG. 7 shows an equivalent circuit diagram of a composite filter inaccordance with another embodiment of the present invention.

FIG. 8 shows frequency characteristics of a composite filter inaccordance with another embodiment of the present invention.

FIG. 9 shows an exploded perspective view of a conventional compositefilter.

FIG. 10 shows an equivalent circuit diagram of a surface acoustic wavefilter used in the conventional composite filter.

DESCRIPTION OF REFERENCE MARKS

-   10 piezoelectric substrate-   11 laminated substrate-   12 surface acoustic wave filter-   13 LC filter-   14 surface acoustic wave resonator-   15 a-15 c capacitor-   16 a-16 c inductor-   17 π-shaped LC filter-   18 antenna terminal-   19 transmission terminal-   20 reception terminal-   21 grounding conductor

PREFERRED EMBODIMENT OF THE INVENTION

An exemplary embodiment of the present invention is demonstratedhereinafter with reference to the accompanying drawings. FIG. 1 shows atop view of a composite filter in accordance with the embodiment of thepresent invention. FIG. 2 shows an exploded perspective view of thecomposite filter. FIG. 3 shows an equivalent circuit diagram of thecomposite filter. FIG. 4 shows frequency characteristics of thecomposite filter.

In FIG. 1-FIG. 3, the composite filter comprises the following elements:

-   -   piezoelectric substrate 10 made from LiTaO₃;    -   laminated substrate 11 made from ceramic baked at a low        temperature, and on which substrate 11 piezoelectric substrate        10 is layered;    -   surface acoustic wave filter 12; and    -   LC filter 13 coupled to surface acoustic wave filter 12.

Elastic surface-wave filter 12 is formed by combining surface acousticwave resonators 14 are placed on piezoelectric substrate 10 such thattheir comb-shaped electrodes oppose to each other. LC filter 13 isformed by combining π-shaped LC filter 17 with capacitor 15 c, wherefilter 17 is formed of capacitors 15 a, 15 b coupled to inductor 16 a ina π-shape, and capacitor 15 c is coupled in parallel to this inductor 16a.

This π-shaped LC filter 17 is configured by connecting respective firstends of capacitors 15 a and 15 b to inductor 16 a, and respective secondends thereof are grounded. Each one of two capacitors 15 a and 15 b isconfigured by placing its comb-shaped electrodes opposing to each otheron piezoelectric substrate, and the opposing direction (A) of thecomb-shaped electrodes differs orthogonally from the opposing direction(B) of comb-shaped electrodes of surface acoustic wave resonator 14.Inductor 16 a is embedded in laminated substrate 11 in a spiral shape.Capacitor 15 c placed in parallel with inductor 16 a is also configuredby placing its electrodes opposing to each other on piezoelectricsubstrate 10, and the opposing direction (A) of its electrodesorthogonally differs from the foregoing opposing direction (B).

Grounding conductor 21 is formed on piezoelectric substrate 10 except atthe place where conductor 21 faces to a top face or an underside ofinductor 16 a embedded in laminated substrate 11, in other words,conductor 21 is placed anywhere on substrate 10 except just above orunder inductor 16 a. Capacitors 15 a and 15 b of π-shaped LC filter 17are coupled to grounding conductor 21.

The structure discussed above allows decreasing a parasitic capacitancegenerated between inductor 16 a and grounding conductor 21, andsuppressing ripples to be generated in the pass band.

The composite filter further includes antenna terminal 18 disposed at afirst end of surface acoustic wave filter 12, and transmission terminal19 disposed at a first portion of a second end thereof via LC filter 13,and reception terminal 20 disposed at a second portion of the second endof surface acoustic wave filter 12. If the reception side wants toattenuate a higher frequency band than the pass band, LC filter 13connected between the reception terminal and the terminal of the surfaceacoustic wave filter will achieve this task. If both of the receptionside and the transmission side want to attenuate a higher frequency bandthan the pass band, LC filter 13 connected between the antenna terminaland the terminal of the surface acoustic wave filter will achieve thistask.

The composite filter discussed above has the frequency characteristicsas shown in FIG. 4, so that the filter can attenuate a large amountoutside the pass band without degrading the waveform of the pass band.Assume that the pass band is 2 GHz, and then the harmonics can beattenuated by approx. 35 dB at 4 GHz (twice of 2 GHz) and 20 dB at 6 GHz(triple of 2 GHz).

Unlike the conventional composite filter formed of planar capacitorelectrodes opposing to each other, capacitors 15 a-15 c can be formed ona highly dielectric piezoelectric-substrate by using a fine patterningsimilar to the surface acoustic wave resonator, so that capacitors 15a-15 c can be placed occupying a smaller area. Particularly the opposingdirection (A) of comb-shaped electrodes of capacitors 15 a-15 c isdifferent from that (B) of comb-shaped electrodes of resonator 14, sothat the comb-shaped electrodes of capacitors 15 a-15 c do not exciteunlike resonator 14, of which comb-shaped electrodes excite. Each one ofcapacitors 15 a-15 c thus can always work as a capacitor regardless offrequencies.

For instance, use of the foregoing composite filter in UMTS (UniversalMobile Telecommunications Systems), a kind of mobile telephones and thethird generation of mobile communication system in Europe, allows theouter dimensions of the filter to be approx. 30 μm×220 μm. Use of theconventional composite filter embedded in laminated substrate, of whichrelative dielectric constant is 7.4 makes its outer dimensions approx.350 μm×330 μm. The occupied area of the composite filter of the presentinvention is thus as small as approx. 1/18 of the conventional one.

As shown in FIG. 5, when capacitor 15 c is replaced with surfaceacoustic wave resonator 14, the resonator can be expressed as anequivalent circuit shown in FIG. 6, namely, capacitor 24 is connected inparallel with a series circuit formed of capacitor 22 and inductor 23.Therefore, an anti-resonant frequency can be created on both the lowfrequency band and the high frequency band with inductor 16 a, which iscoupled in parallel to resonator 14, without changing a capacitance ofsurface acoustic wave resonator 15 while the capacitance of anequivalent circuit of resonator 14 is kept equivalent to that ofcapacitor 15. At the same time, the anti-resonant frequency can bechanged with a value of the inductor without changing the resonancefrequency. The equivalent circuit of resonator 14 thus can produce asimilar effect to that of capacitor 15 c in the vicinity of the passband. On top of that, signals in a lower band than the pass band can beattenuated by anti-resonant frequency created by the inductor on thelower band side.

Laminated substrate 11 includes another grounding conductor (not shown)embedded therein than grounding conductor 21 formed on piezoelectricsubstrate 10, and connecting of these two grounding conductors to eachother allows producing inductors 16 b and 16 c between these conductorsas shown in FIG. 7. Resonance between inductors 16 b, 16 c andcapacitors 15 a, 15 b thus can be positively used, so that the values ofcapacitors 15 a, 15 b and capacitance that resonator 14 has, andrespective values of inductors 16 a-16 c can be designed for theseelements to form a polarized low-pass filter.

The composite filter discussed above has the frequency characteristicsas shown in FIG. 8, and can attenuate a large amount of signals outsidethe pass band without degrading the waveform of the frequencycharacteristics in the pass band. For instance, assume that the passband is 2 GHz, comparing with a typical surface acoustic wave filter,the foregoing composite filter can attenuate the signals outside thepass band by approx. 40 dB at 4 GHz (twice of 2 GHz), and approx. 30 dBat 6 GHz (triple of 2 GHz).

From the standpoint of capacitance, the comb-shaped electrodes ofcapacitors 15 a-15 c can be covered with dielectric such as SiO₂ or SiNfor increasing their capacitances, and this preparation can alsosuppress the dispersion of the capacitances due to changes intemperature.

Inductor 16 a can be formed of strip lines having equivalent dielectricto that of the equivalent circuit of resonator 14 at a given frequency,or formed of chip inductors for obtaining a higher Q value, or formed onpiezoelectric substrate 10 instead of embedded in laminated substrate 11in order to downsize the filter.

INDUSTRIAL APPLICABILITY

The composite filter of the present invention can be used incommunication devices such as mobile phones.

1. A composite filter comprising: a surface acoustic wave filter, and aLC filter coupled to the surface acoustic wave filter, wherein thesurface acoustic wave filter is formed by combining surface acousticwave resonators of which comb-shaped electrodes oppose to each other ona piezoelectric substrate, wherein the LC filter includes a π-shaped LCfilter formed of a first capacitor coupled to an inductor in a π-shapeand a second capacitor coupled in parallel to the inductor, wherein atleast one of the capacitors is formed of comb-shaped electrodes opposingto each other on the piezoelectric substrate, and the opposing directionof the comb-shaped electrodes differs from an opposing direction of thecomb-shaped electrodes of the surface acoustic wave resonators.
 2. Thecomposite filter of claim 1, wherein the piezoelectric substrate islayered on a dielectric substrate and the inductor is embedded in thedielectric substrate.
 3. The composite filter of claim 2, wherein thepiezoelectric substrate is provided with a grounding conductor, which isdisposed at a place not confronting a top face or an underside of theinductor, and the capacitor of the π-shaped LC filter is coupled to thegrounding conductor.
 4. The composite filter of claim 1, whereincomb-shaped electrodes forming the first capacitor and the secondcapacitor are covered with dielectric.